GCC optimization bug?

// Excerpt from display.h:
#define BASE_REG 0x4000000

#define DISPLAY_BG_MODE_0        (0)
#define DISPLAY_BG_MODE_1        (1)
#define DISPLAY_BG_MODE_2        (2)
#define DISPLAY_BG_MODE_3        (3)
#define DISPLAY_BG_MODE_4        (4)
#define DISPLAY_BG_MODE_5        (5)

#define DISPLAY_MODE_GBA         (0 << 3)
#define DISPLAY_MODE_CGB         (1 << 3)

#define DISPLAY_SELECT_FRAME_0   (0 << 4)
#define DISPLAY_SELECT_FRAME_1   (1 << 4)

#define DISPLAY_HBLANK_FREE      (1 << 5)
#define DISPLAY_OBJ_VRAM_MAPPING (1 << 6)
#define DISPLAY_FORCED_BLANK     (1 << 7)

#define DISPLAY_SHOW_BG0         (1 << 8)
#define DISPLAY_SHOW_BG1         (1 << 9)
#define DISPLAY_SHOW_BG2         (1 << 10)
#define DISPLAY_SHOW_BG3         (1 << 11)
#define DISPLAY_SHOW_OBJ         (1 << 12)
#define DISPLAY_SHOW_WIN0        (1 << 13)
#define DISPLAY_SHOW_WIN1        (1 << 14)
#define DISPLAY_SHOW_WIN_OBJ     (1 << 15)

typedef struct display_control_reg {
    union {
        unsigned short raw;
        struct __attribute__((packed)) {
            unsigned bg_mode              : 3;
            unsigned cgb_gba              : 1;
            unsigned display_frame_select : 1;
            unsigned hblank_free          : 1;
            unsigned obj_vram_mapping     : 1;
            unsigned forced_blank         : 1;
            unsigned display_bg0          : 1;
            unsigned display_bg1          : 1;
            unsigned display_bg2          : 1;
            unsigned display_bg3          : 1;
            unsigned display_obj          : 1;
            unsigned display_win0         : 1;
            unsigned display_win1         : 1;
            unsigned display_win_obj      : 1;
        };
    };
} display_control_reg_t;

#define display_control_register (*(display_control_reg_t *)BASE_REG)

// excerpt from main.c
display_control_register.raw = 0;
display_control_register.bg_mode = 3;
display_control_register.display_bg2 = 1;

// code generated by gcc
08000340 <main>:
 8000340:   b500        push    {lr}
 8000342:   2380        movs    r3, #128    ; 0x80
 8000344:   04db        lsls    r3, r3, #19
 8000346:   2200        movs    r2, #0
 8000348:   801a        strh    r2, [r3, #0]
 800034a:   7859        ldrb    r1, [r3, #1]
 800034c:   2204        movs    r2, #4
 800034e:   430a        orrs    r2, r1
 8000350:   705a        strb    r2, [r3, #1]
 8000352:   23c0        movs    r3, #192    ; 0xc0
 8000354:   4a03        ldr r2, [pc, #12]   ; (8000364 <main+0x24>)
 8000356:   04db        lsls    r3, r3, #19
 8000358:   211f        movs    r1, #31
 800035a:   8019        strh    r1, [r3, #0]
 800035c:   3302        adds    r3, #2
 800035e:   4293        cmp r3, r2
 8000360:   d1fb        bne.n   800035a <main+0x1a>
 8000362:   e7fe        b.n 8000362 <main+0x22>
 8000364:   06012c00    .word   0x06012c00